How councils apply the BRE daylight and sunlight guidance

The BRE guidance on site layout planning for daylight and sunlight is advice, not policy, and it says so itself. Councils apply it because local plans point to it and because nothing better exists, but the numerical guidelines in it are described in the document as flexible values to be interpreted according to context. A scheme that misses a guideline is not automatically unacceptable, and a scheme that meets every guideline is not automatically approved.

That distinction decides how you should write the assessment. The numbers are the start of the argument.

What the guidance is and what status it carries

The current third edition, published in 2022, sets out methods for assessing daylight and sunlight to existing buildings, to proposed buildings and to open space. It is published by BRE rather than by government, which means it has no statutory force. Its weight comes from adoption: many local plans and design supplementary planning documents refer to it by name, and inspectors routinely use it as the common yardstick.

The 2022 edition changed the assessment of new interiors, moving away from average daylight factor towards the illuminance-based targets in BS EN 17037. Reports still arrive using the older method, and an officer comparing two schemes assessed on different bases has a genuine problem. State the edition and the method on the first page.

Tests for existing neighbours

For daylight to an existing window, two measures do the work. Vertical sky component measures the proportion of sky visible at the centre of the window on the outside face, with a guideline value of 27 per cent and a further test comparing the proposed value against 0.8 times the existing value. Daylight distribution, sometimes called the no sky line, measures the area of the working plane inside the room that can still see the sky, again against 0.8 times the former area.

For sunlight, annual probable sunlight hours are assessed for windows within 90 degrees of due south, with separate annual and winter measures.

For open space and gardens, the overshadowing check looks at whether at least half the area receives at least two hours of sunlight on 21 March.

The Daylight and Sunlight Assessment reports all of these, and a separate Overshadowing Study is sometimes commissioned where amenity space rather than windows is the issue.

Tests for the rooms you are proposing

Assessing your own scheme is a different exercise. The question is whether the rooms you are creating will be adequately lit, judged against target illuminance levels for the room type, achieved across a defined proportion of the space for at least half the daylight hours.

Room use drives the target, so the labelling on the plans matters more than architects expect. A room drawn as a study and assessed as a bedroom invites an objection. Where an open-plan living, kitchen and dining space is proposed, the kitchen target governs a larger area than many designers assume, and deep plans with a single window wall struggle.

Balconies are the recurring culprit. A projecting balcony above a window removes a large part of the visible sky and can push an otherwise sound room below target. Recessed balconies, cut-outs above the window head and glass balustrades all help.

How officers actually use the numbers

In my experience, officers read a daylight report in three passes. First, how many windows or rooms fall below the guideline. Second, by how much. Third, what those rooms are and who lives in them.

A 24 per cent vertical sky component in an inner urban street where the existing value was 26 per cent is a different case from a fall from 31 per cent to 18 per cent in a suburban cul-de-sac. The guidance anticipates exactly that, and it explicitly contemplates lower target values in dense urban areas where a higher density is the plan's own objective.

What officers dislike is a report that presents 40 per cent of rooms as non-compliant, offers no analysis, and leaves them to write the amenity paragraph themselves. Where results fall short, say which rooms, what they are used for, what the existing condition was, why the shortfall arises, and what the design does about it.

Where flexible application is expected and where it is not

Flexibility is strongest where the existing condition is already constrained, where the local plan sets a density the guidance's suburban assumptions do not fit, and where the affected rooms are non-habitable or secondary.

It is weakest in three situations. Where the affected property is a family home with a single living room losing most of its light. Where the loss is caused by a design decision with an obvious alternative, such as a blank flank wall pushed to the boundary. And where the proposal creates poor conditions in its own new homes, since an authority cannot flex a standard on behalf of occupiers who have no say.

The London Plan expects daylight and sunlight to be assessed with regard to context rather than mechanically, and boroughs vary in how they apply that. Check the borough's own supplementary guidance before assuming.

Presenting results an officer can follow

A usable report contains a small number of things done well.

• A location plan numbering every assessed window and room, cross-referenced to the results tables
• A statement of the software, the method, the guidance edition and the assumptions on reflectance and glazing transmittance
• Existing and proposed values side by side, with the ratio, rather than proposed values alone
• Separate tables for daylight, sunlight and overshadowing, not one merged sheet
• A short written analysis of every failure, room by room
• A clear statement of what was modelled as existing, including any consented but unbuilt schemes nearby

Model the existing condition honestly. Assessing against a permitted but unbuilt neighbouring scheme, or against a hypothetical fully developed site, is a legitimate sensitivity test and an illegitimate baseline. Present it as the former.

Misuses that damage a case

Four recur often enough to name.

Assessing rooms that are not habitable, then reporting a high compliance rate. Bathrooms, halls and storage inflate the percentage and officers know it.

Quoting the 27 per cent vertical sky component figure as a pass mark. It is a guideline drawn from low-density suburban conditions, and the guidance itself says so.

Using average daylight factor because the older edition is familiar, then defending it at appeal against a report using the current method.

Reporting percentages of windows meeting the guideline without saying how many rooms are affected or which. Ten failures across two flats is a different case from ten failures across ten flats.

Illustrative example: a five-storey infill on a corner site in a dense borough. Twelve neighbouring windows fall below 0.8 times the existing vertical sky component, all in the rear elevation of a converted terrace. The report identifies eight as secondary windows to dual-aspect rooms, two as kitchens with a second window, and two as single-aspect bedrooms. The design pulls the top floor back three metres above those two bedrooms. The officer's report deals with the issue in a paragraph, because the assessment already had.

Councils usually expect a suitably qualified author for this work, and professional review before submission is strongly advised. Reviewers can be found through the professional directory.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.